(Not Applicable)
1. Field of the Invention
The invention relates to an apparatus and method for measuring the properties of materials. More specifically, the invention relates to an apparatus and method for the in situ, real-time monitoring and control of properties and characteristics of a material.
2. Description of the Relevant Art
For at least the last four decades, carbon fibers have been used as a substitute for steel, aluminum, titanium, and glass fibers, among others. Carbon fibers are an ideal material for use in a wide range of applications due to their strength and stiffness, light weight, high fatigue resistance and vibration damping, corrosion resistance, good friction and wear qualities, low thermal expansion, and thermal and electrical conductivity. Thus, fabrication and design using carbon fibers offers a degree of versatility that is not available when using other materials.
Carbon fibers are manufactured conventionally through the controlled pyrolysis, or chemical change through heating, of a precursor material. This precursor material includes rayon (or regenerated cellulose) fibers, pitch (petroleum and coal-tar)-based fibers, and polyacrylonitrile (PAN) fibers, among others. This pyrolysis has continuous sequential process stages, and each process stage is identified by a unique set of process conditions. Carbon fibers are manufactured to certain specifications, which are dependent on their ultimate use. The transformation of precursor materials to carbon fibers during the manufacturing process must be monitored in some way to ensure that these specifications are met. Current quality control of carbon fiber manufacturing is limited to non-real time analysis. These current quality control methods require the physical removal of a sample from the carbon fiber after the completion of each specific process stage. These samples are then taken to a laboratory for determining whether that carbon fiber falls within its required specification range (e.g., density, electrical resistivity, mechanical strength, etc.). Once an analysis of the sample is made, process control decisions are made to either maintain the process, or to regulate the process and bring the unprocessed precursor material into the desired specification range.
This current quality control method is both time consuming and inefficient. In general, a carbon fiber production line consists of multiple carbon fiber tows (or strandsxe2x80x94up to around 300), being produced at a linear processing speed up to approximately 1 linear foot per second depending on specific products and their quality specifications. There is a considerable passage of time (from 0.5 to 3 hours) between the initial removal of carbon fiber samples from the production line and any necessary changes which need to be made to the production process to ensure that future tows fall within the desired specification range.
Thus, the time spent sampling and analyzing processed carbon fibers results in the production of quantities of carbon fibers that fall outside of the desired specification range. These unacceptable carbon fibers are then either fully rejected or diverted to a secondary market, resulting in either an absolute loss or reduction of profit to the carbon fiber producer (and transitively a high cost to the end user of those carbon fibers that do fall within a desired specification range). It is therefore desirable to be able to perform real-time monitoring for unacceptable carbon fibers during the production process, so that a producer can immediately determine whether the properties of a given carbon fiber tow fall within a desired specification range. Such immediate knowledge allows the producer to modify the production process earlier, resulting in the reduced production of unacceptable carbon fibers.
Although microwave energy has not been used to make in situ, real-time measurements of the properties of carbon fiber tows during their production, it has been used to measure the properties of various other materials. For example, U.S. Pat. No. 5,648,038 to Fathi et al. discloses the use of microwave energy to measure an entire material inside a chamber by generating variable frequency microwave energy and using the detected power reflection for each one of the generated variable frequencies to determine certain properties of that material. Fathi et al. does not disclose the use of microwave signals to measure the dielectric properties of a given material, rather it discloses the measurement of the reflection of microwave signals generated in a microwave cavity. Furthermore, Fathi et al. defines measurement as occurring in a microwave cavity having multiple modes, which requires sweeping over a wideband frequency range in order to achieve uniformity in the measurement of the desired material.
Another example of the use of microwave energy to measure material properties is disclosed in U.S. Pat. No. 4,904,928 to Lewis. Lewis discloses the measuring of material properties through the difference of the frequency of oscillation of least two identical resonant modes having substantially the same resonant frequencies but orthogonal field orientations relative to one another as those modes are oscillating in a symmetrical microwave cavity. Lewis discloses the geometrical (diameter) measurement of a given material by monitoring changes in frequency separation between modes which are by variations in amount of the low loss dielectric material present. Lewis does not disclose the measurements of possible variations in the fiber diameter by monitoring changes in the transmitted signal level.
An example of the measurement of the dielectric properties of a material is disclosed in U.S. Pat. No. 5,219,498 to Keller et al. Keller et al. discloses the use of a low frequency signals and measures capacitance and dielectric loss between metallic electrodes embedded in the composite matrix of the measured resins or composites. Keller et al. does not disclose the measurement of the dielectric properties of a material without the use of an embedded metallic electrode, such as by a microwave signal.
Thus, none of these references address the problem of using a resonance cavity to determine the dielectric properties of a given carbon fiber tow in order to monitor changes in the specific degree of carbonization or graphitization characteristics of that carbon fiber tow during it production process.
It is an object of the invention to provide an apparatus and a method for monitoring the purity of a material composition.
It is an object of the invention to provide an apparatus and a method for monitoring the quality of a material composition.
It is an object of the invention to provide an apparatus and a method for monitoring the composition of a material composition.
It is an object of the invention to provide an apparatus and a method for monitoring the moisture content of a material composition.
It is an object of the invention to provide an apparatus and a method for the in situ, real-time monitoring of the carbon fiber production process.
It is another object of the invention to provide an apparatus and a method for the in situ, real-time control of the carbon fiber production process.
It is still another object of the invention to provide a method and apparatus for producing carbon fiber tows falling within a given specification range that consumes a reduced amount of precursor material.
It is yet another object of the invention to provide a method and apparatus for producing carbon fiber tows falling within a given specification range in a manner consuming reduced resources, including time and energy, among other things.
It is a further object of the invention to provide a method and apparatus for using microwave energy to measure the properties of a carbon fiber tow during the production process.
It is still a further object of the invention to provide a method and apparatus for using microwave energy to monitor the properties of a carbon fiber tow during the production process.
It is an additional object of the invention to provide a method and apparatus for using microwave energy to control the properties of a carbon fiber tow during the production process.
These and other objects of the invention are achieved by the subject method which comprises applying a frequency signal, such as microwave or RF energy, to make an in situ, real-time measurement of the properties of a material composition, such as a carbon fiber tow, during or after the production of the material. The step of measuring the properties of the material composition can be accomplished in any manner and includes applying the frequency signal, such as microwave or RF energy, to a measurement applicator, such as a resonance cavity or a parallel plate dielectric measurement device, placing or continuously feeding the material composition, such as the carbon fiber tow, in or through the resonance cavity, measuring changes in the microwave field applied to the resonance cavity caused by the portion of material composition present in the resonance cavity, and relating the changes in the microwave field applied to the resonance cavity caused by the portion of material composition, such as the carbon fiber tow, present in the resonance cavity to properties of that portion of material composition, such as purity, composition, moisture content or fiber density, among others.
The microwave energy applied to the resonance cavity, and to the material composition fed through the resonance cavity, should be of low enough power to prevent changes in the structure of the material composition being monitored. For example, when the material composition is a carbon fiber tow, the microwave energy applied to the resonance cavity should be of low enough power to allow for the continued oxidation, carbonization or graphitization of the carbon fiber tow. The microwave or RF band energy applied to the resonance cavity preferably has a fixed frequency. Also, the resonance cavity preferably has a single mode.
Any intrinsic property related to the moisture content, purity, composition or absorptivity of the material composition, and specifically any changes or variations of these properties, along the length of the monitored material composition can be selected to measure the characteristics of the material composition. For example, absorptivity can be related to the specific, well defined state of a carbon fiber tow during its processing. Furthermore, the association between the values of any purity, moisture content, composition or absorptive intrinsic property of the monitored material composition, such as the loss tangent of the material composition, and the moisture, purity, composition or morphology of the material composition must be known. These associating values may be formulated into a baseline containing the interrelation between any of the dielectric properties of the material compositions, such as the loss tangent of the material composition, and the corresponding characteristics of the material composition, such as the purity, composition or morphology (i.e., intrinsic density), among other characteristics.
In another embodiment of the present invention, an apparatus for monitoring properties or characteristics of materials is disclosed. This apparatus includes an applicator for applying a frequency signal to an application zone; a placement device for placing a material in the application zone; a measurer for measuring a property value resulting from said applicator applying the frequency signal to the application zone prior to said placement device placing the material in the application zone and for measuring changes in the property value of the application zone caused by said placement device placing the material in the application zone; and a processor for relating said changes in the property value of the application zone caused by said placement device placing the material in the application zone to a desired characteristic of the material. The placement device can also be a feeder that feeds the material through the application zone. When the feeder is used, the measurer continuously measures the property values of the portions of material being fed through the application zone.
According to another embodiment of the invention, an apparatus for producing carbon fibers may comprise a precursor filament transport for transporting a precursor filament through the carbon fiber production process; a pretreater for preparing the precursor material for the oxidation process; an oxidizer for inducing the stabilization and oxidation of the precursor filament; a carbonizer for inducing the carbonization of an oxidized precursor filament into carbon filaments; a graphitizer for inducing the graphitization of the carbon filaments; a treater for treating the surface of the carbon fibers which have graphitized; a post-production device for sizing and/or drying produced carbon fiber tows; a packager for packaging the carbon fibers; and a material composition monitor for monitoring the morphology of a filament during the carbon fiber production process. The transport need not continuously transport the precursor filament through the carbon fiber production process; the process can be incremental or performed in batches.
In this embodiment of the invention, the material composition monitor comprises a measurement applicator, such as a resonant cavity among other things, in which certain measurements, such as changes in dielectric properties, can be made; a carbon fiber tow which is placed or continuously fed through the resonance cavity; a signal generator for applying frequency signals, such as microwave energy, through a device, such as a waveguide or coaxial cable, to the measurement applicator; and a microwave measuring device connected to the resonance cavity. Preferably, the material composition monitor also comprises a processor for correlating the output of the microwave measuring device to the corresponding characteristics of the carbon fiber tow which is placed in, or fed through, the resonance cavity.
In the present invention, the measurement apparatus is preferably a resonance cavity. The microwave measuring device measures the changes in frequency signal, which is preferably a microwave field, applied to the resonance cavity caused by the placing or continuous feeding of the portion of the carbon fiber tow through the resonance cavity. In the present invention, the change in the microwave field applied to the resonance cavity is preferably the change in the electric quality factor (Q) of the resonance cavity. In the present invention, the resonance cavity is preferably an empty resonance cavity with a known electric quality factor. In the present invention, the processor preferably correlates the change in the electric quality factor of the resonance cavity caused by the continuous feeding of the carbon fiber tow through the resonance cavity to the characteristics of the carbon fiber tow, such as intrinsic density, among others.